601 FXUS65 KTFX 041550 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 945 AM MDT Sat May 4 2013 .UPDATE... Latest radar/satellite imagery showing broad area of mostly cloudy to overcast skies thru central and southwest MT with some scattered areas of light rain from southern/eastern Cascade County eastward to central Fergus County. Forecast still on track for easterly flow aloft to bring in additional moisture this aftn/eve and generate scattered rain showers over all parts of the forecast area except the Hiline counties which should remain mostly dry. Only slight forecast grid adjustments this morning, mainly to reduce precip chances and cloud cover along the Hiline and increase them over the Little/Big Belt Mtns. Waranauskas && .DISCUSSION... Today through Monday...Deep moisture combined with a deep and persistent northerly upslope flow will keep at least scattered showers around north central and southwest Montana today. The shortwave trough that brought widespread precipitation to the area yesterday and overnight has begun cutting off into an area of low pressure as it moves west across the northern Great Basin. As a result, the aforementioned moisture is wrapping around the upper low from the Great Plains and into Montana. With the upper low still in the vicinity, the greatest potential for measurable precipitation will be this morning. Showers this afternoon will be more scattered due to weak instability, along with possible isolated thunderstorms in far southwest Montana in the vicinity of the cold front that moved through the area yesterday. Clouds and showers will diminish overnight, which may allow patchy fog to form along the east slopes of the Rockies and in the southwest Montana valleys. Lingering easterly winds across the eastern half of the area will likely hinder fog development there. As the upper low gradually moves west toward the California coast through the remainder of the period, weak high pressure aloft will build into the area, for mostly dry and warming conditions. Temperatures will warm from near seasonal averages today to 70s at most lower elevations by Monday. With this warming, afternoon/evening instability will develop for Sunday and Monday. However, moisture be fairly limited to southwest Montana, so have kept the mention of afternoon and evening showers and thunderstorms only in that area. Coulston Monday night through Saturday...Monday night through Tuesday will see an east-west oriented upper ridge across southern Canada with an upper low over the southwest U.S. The upper ridge should keep the north dry while weak instability over southwest Montana could help in producing scattered showers and isolated thunderstorms. Tuesday night and Wednesday the upper ridge will shift south as a weather disturbance aloft moves to central Canada. Associated with this disturbance a slightly cooler Canadian airmass will spread into the forecast area. The main precipitation associated with disturbance should stay north of the border however by Wednesday instability could spread farther north so will continue with the idea of having at least isolated showers over central Montana. Again there will be a threat of thunderstorms over southwest Montana. With the surge of Canadian air highs Wednesday should be 5 to 10 degrees cooler than those of Tuesday. Thursday models agree on a northwest flow aloft but do not agree on precipitation details so will go with the idea of scattered showers for the mountains and isolated showers for the lower elevations. There will also be a small threat for thunderstorms over all but north central Montana. Friday into Saturday northwest flow aloft is expected to increase. A Canadian cold front and associated precipitation will spread south over Montana. The GEM and ECMWF model runs are faster than the GFS with the precipitation bringing it into Montana Friday while the GFS brings in the precipitation later Friday night. The GFS has lingering precipitation into Saturday while the other models tend to diminish precipitation. Due to model differences will not go above the chance category for the threat of precipitation at this point. Blank && .AVIATION... UPDATED 1120Z. For this morning confidence is on the low side with regard to whether conditions will be MVFR or VFR over southwest Montana. Although not mentioned in the tafs there could be a period of MVFR ceilings and possibly MVFR visibilities in rain showers at KBZN and KHLN. Had higher confidence for a period of MVFR conditions at KGTF so have a tempo group. KCTB and KHVR should not see MVFR conditions. Confidence in conditions at KLWT is low as could see anywhere from LIFR to VFR conditions. By this afternoon expect VFR conditions at all TAF sites. There will be scattered showers into this afternoon with snow showers over the mountains and areas of mountain obscurement. Conditions will improve late this afternoon into this evening as showers end and clouds decrease. Expect widespread clear skies by midnight. Blank && .PRELIMINARY POINT TEMPS/POPS... GTF 60 36 68 40 / 40 10 0 0 CTB 62 36 67 39 / 20 0 0 0 HLN 60 35 67 39 / 50 20 10 0 BZN 60 31 67 34 / 50 10 10 10 WEY 53 26 59 27 / 40 10 20 20 DLN 58 32 65 38 / 40 20 20 20 HVR 66 36 71 39 / 30 10 0 0 LWT 57 33 63 37 / 50 10 10 0 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...COULSTON LONG TERM...BLANK AVIATION...mpj weather.gov/greatfalls